The present invention generally relates to liquid crystal display devices and more particularly to a liquid crystal display device operating in a so-called VA (Vertically Aligned) mode in which liquid crystal molecules having a negative dielectric anisotropy or positive dielectric anisotropy are aligned generally perpendicularly to a panel surface of the liquid crystal display device.
Liquid crystal display devices are used as a display device of various information processing apparatuses such as a computer. Liquid crystal display devices, having a compact size and consuming little electric power, are particularly suitable for application in portable information processing apparatuses. On the other hand, use of such liquid crystal display devices also in a fixed-type information processing apparatus such as a desktop-type computer, is also studied.
Conventional liquid crystal display devices generally use a so-called TN (Twisted Nematic)-mode construction in which p-type liquid crystal molecules having a positive dielectric anisotropy are aligned horizontally between a pair of mutually opposing panel substrates, wherein the liquid crystal molecules adjacent to one panel substrate and the liquid crystal molecules adjacent to the other panel substrate are aligned in respective directions crossing with each other perpendicularly.
In such a TN-mode liquid crystal display device, various liquid crystals are already developed, and the liquid crystal display device can be fabricated by a well-established process with low cost.
On the other hand, a TN-mode liquid crystal display device has a drawback in realizing a high contrast representation of images. It should be noted that a TN-mode liquid crystal display device provides a black representation by causing the liquid crystal molecules to align vertically to the principal surface of the panel substrate by applying a driving electric field, while the liquid crystal molecules immediately adjacent to the panel substrate tend to maintain the horizontal alignment even when the driving electric field is applied. Thereby, the birefringence associated with such horizontal liquid crystal molecules allows a passage of light even in the activated state in which the passage of light through the liquid crystal layer should be interrupted completely. Thus, there occurs a leakage of light or coloring of the panel when an attempt is made in a TN-mode liquid crystal display device to display a white image on a black background (so-called xe2x80x9cnormally black modexe2x80x9d) as is commonly adopted in a CRT display device. Thus, the black representation becomes worse than that of a xe2x80x9cnormally white mode,xe2x80x9d in which black images are displayed on a white background, because of dispersion. This is the reason why conventional TN-mode liquid crystal display devices are operated in the normally white mode.
A VA-mode liquid crystal display device is a liquid crystal display device in which liquid crystal molecules having a negative or positive dielectric anisotropy are confined between a pair of panel substrates in a state that the liquid crystal molecules are aligned in a direction generally perpendicular to the principal surface of the panel substrates in a non-activated state of the liquid crystal display device. Thus, a light passes through a liquid crystal layer in such a liquid crystal display device without changing the polarization plane thereof in the non-activated state of the liquid crystal device, and the light is effectively interrupted by a pair of polarizers disposed at both sides of the liquid crystal layer in a crossed Nicol state. In such a VA-mode liquid crystal display device, therefore, it is possible to achieve a near-ideal black representation in the non-activated state of the liquid crystal display device. In other words, such a VA-mode liquid crystal display device can easily achieve a very high contrast representation not possible by a TN-mode liquid crystal display device.
In an activated state of a VA-mode liquid crystal display device, it should be noted that the liquid crystal molecules are aligned generally parallel to the panel substrates due to the electric field applied to the liquid crystal molecules, and a rotation is induced in the polarization state of an incident optical beam. Thereby, the liquid crystal molecules thus activated show a 90xc2x0-twist between the first panel substrate and the second panel substrate.
The VA-mode itself has been known for a long time. Further, there exists a report about the property of a liquid crystal having a negative dielectric anisotropy (D. de Rossi, J. Appl. Phys. 49(3), March, 1978).
On the other hand, it has been thought conventionally that a VA-mode liquid crystal display device cannot provide the quality of representation comparative to that of a TN-mode liquid crystal display device, in terms of response time, viewing-angle characteristics, voltage retention (or voltage holding ratio), and the like. Thus, little effort has been made so far for realizing a practical liquid crystal display device using a VA-mode liquid crystal. Particularly, it has been believed that construction of an active-matrix liquid crystal display device that uses thin-film transistors (TFT) is very difficult.
As a VA-mode liquid crystal can provide a contrast ratio superior to that of a conventional CRT (cathode-ray tube) display device, it is predicted that the major target of such a VA-mode liquid crystal display device would be to replace conventional CRT display devices. In order to achieve this target, however, it is particularly necessary to improve the viewing-angle characteristics of the display device, in addition to usual requirements of increasing the display area and improving the response.
Japanese Laid-open Patent Publication 62-180326 describes a VA-mode liquid crystal display device in which a liquid crystal layer formed of liquid crystal molecules having a negative dielectric anisotropy, is confined between a pair of glass substrates such that the liquid crystal molecules align generally perpendicularly to the substrate surface in a non-activated state thereof in which no drive voltage is applied across the glass substrates. The reference further describes a construction to cause a 90xc2x0-twist for the liquid crystal molecules in the direction generally parallel to the substrate surface in the activated state thereof in which the drive voltage is applied across the substrates. Further, the reference teaches to dispose a polarizer and an analyzer at respective outer sides of the glass substrates such that respective optical absorption axes intersect perpendicularly with each other.
Japanese Laid-open Patent Publication 3-5721, on the other hand, describes a VA-mode liquid crystal display device in which a liquid crystal layer formed of liquid crystal molecules having a negative dielectric anisotropy, is confined between a pair of substrates, In the above noted reference, the liquid crystal layer has a retardation set in a range between 0.6 xcexcm and 0.9 xcexcm, and first and second birefringence media are disposed at both sides of a liquid crystal panel thus formed. Further, the reference teaches to provide a polarizer and an analyzer at respective outer sides of the foregoing birefringence media so as to cross the respective optical absorption axes perpendicularly. Further, the reference teaches to set the optical absorption axes so as to form a 45xc2x0 angle with respect to the optical axes of the birefringence media.
Further, Japanese Laid-open Patent Publication 5-113561 describes a photo-conduction type liquid crystal light valve, wherein the reference teaches the use of a liquid crystal of negative dielectric anisotropy for a liquid crystal layer provided adjacent to a photo-conduction layer, such that the liquid crystal molecules align generally perpendicularly to the electrode surface in the non-activated state of the liquid crystal layer. Further, the reference teaches a feature to set the retardation of the liquid crystal layer to be 0.3 xcexcm or more.
Further, Japanese Laid-open Patent Publication 5-113561 describes a VA-mode liquid crystal display device that includes optical compensation means having a negative optical activity in addition to a pair of substrates that confine a liquid crystal layer of liquid crystal molecules having a negative dielectric anisotropy therebetween, wherein the liquid crystal display device further includes first and second quarter-wavelength phase shift plates such that the first phase shift plate has a positive optical activity and an optical axis parallel to the substrates and such that the second phase shift plate has a negative optical activity and an optical axis parallel to the optical axis of the first phase shift plate. The liquid crystal display device of the reference further includes a polarizer and an analyzer in a crossed Nicol state such that the polarizer and the analyzer sandwich the foregoing construction therebetween.
However, such conventional VA-mode liquid crystal devices, while capable of providing a contrast ratio exceeding the contrast ratio achieved by the conventional TN-mode or STN-mode liquid crystal display devices, cannot provide response, viewing-angle characteristics, brightness and colorless representation required for a desktop display device.
Accordingly, it is a general object of the present invention to provide a novel and useful liquid crystal display device wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide a VA-mode liquid crystal display device that uses a liquid crystal having a negative or positive dielectric anisotropy, in which the liquid crystal display device is optimized with respect to response, viewing-angle and contrast of representation.
Another object of the present invention is to provide a liquid crystal display device, comprising:
a first substrate and a second substrate sandwiching a liquid crystal layer therebetween;
a first polarizer disposed adjacent to said first substrate at a side opposite to a side of said first polarizer facing said liquid crystal layer, with a first gap between said first polarizer and said first substrate;
a second polarizer disposed adjacent to said second substrate at a side opposite to a side of said second polarizer facing said liquid crystal layer, with a second gap between said second polarizer and said second substrate; at least one of said first and second gaps including therein a first retardation film having a positive optical anisotropy and a second retardation film having a negative optical anisotropy, such that said first retardation film is disposed closer to said liquid crystal layer with respect to said second retardation film.
According to the present invention, a wide viewing-angle is realized in a VA-mode liquid crystal display device by disposing the first and second retardation films adjacent to the liquid crystal layer.
Another object of the present invention is to provide a liquid crystal display device, comprising:
a first substrate and a second substrate sandwiching a liquid crystal layer therebetween;
a first polarizer disposed adjacent to said first substrate at a side opposite to a side of said first polarizer facing said liquid crystal layer, with a first gap between said first polarizer and said first substrate;
a second polarizer disposed adjacent to said second substrate at a side opposite to a side of said second polarizer facing said liquid crystal layer, with a second gap between said second polarizer and said second substrate;
at least one of said first and second gaps including therein an optically biaxial retardation film.
According to the present invention, a wide viewing-angle can be realized by using the optically biaxial retardation film adjacent to the liquid crystal layer.
Another object of the present invention is to provide a liquid crystal display device, comprising:
first and second substrates disposed substantially parallel to each other, said first substrate having a first principal surface at a side thereof facing said second substrate, said second substrate having a second principal surface at a side thereof facing said first substrate;
a first electrode pattern provided on said first principal surface of said first substrate;
a second electrode pattern provided on said second principal surface of said second substrate;
a first molecular orientation film disposed on said first principal surface of said first substrate so as to cover said first electrode pattern;
a second molecular orientation film disposed on said second principal surface of said second substrate so as to cover said second electrode pattern;
a liquid crystal layer confined between said first and second molecular orientation films;
said liquid crystal layer containing liquid molecules such that a major axis of said liquid crystal molecule aligns generally perpendicularly to at least one of said first and second principal surfaces;
said liquid crystal layer having a retardation of about 80 nm or more but below about 400 nm.
According to the present invention, it becomes possible to construct the liquid crystal display device to have a wide viewing-angle, high response speed and a colorless, high-contrast representation.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.